Special features and limitations of a conventional variable eyepiece design and a gradient-index design example are compared. Optical performance and manufacturing aspects are discussed. The influence of gradient-index lenses on the different aberrations is shown. A rough cost estimation of both -- classical and gradient-index design -- is presented.
"Gradient-index design example for a variable eyepiece", Proc. SPIE 3110, 10th Meeting on Optical Engineering in Israel, (22 September 1997); doi: 10.1117/12.281344; https://doi.org/10.1117/12.281344